Highly Active Chiral Zinc Catalysts for Immortal Polymerization
of β‑Butyrolactone Form Melt Processable Syndio-Rich
Poly(hydroxybutyrate)
- Publication date
- Publisher
Abstract
Highly
crystalline poly(hydroxybutyrate) suffers from high melting
point and entanglement molecular weight. This leads to low melt strength,
limits processing through regular techniques, and precludes many applications.
In this work we report a series of racemic and enantiopure zinc catalysts
supported by variously substituted diaminophenolate ancillary ligands
which form high melt strength PHBs with different molecular weights
and microstructure. These complexes are active for the highly controlled
polymerization of β-butyrolactone (BBL); some can polymerize
2000 equiv of BBL in less than 30 min. Changing the steric bulk of
the ligand forms PHBs of varied syndiotacticity (<i>P</i><sub>r</sub> = 0.75 to 0.55). These are highly robust systems capable
of polymerizing an unprecedented 20000 equiv of BBL in the presence
of 5000 equiv of benzyl alcohol. Thermorheological investigations
reveal that the synthesized PHBs have surprisingly high melt strength
at above the melting point. For processable PHBs, high density of
entanglements and relatively low crystallinity are crucial. We show
that the best PHBs should have high molecular weight and moderate
syndiotacticity